Any new filter compared to an old dirty one will show at least some gains. But new versus new? You'd be lucky to pick up 2-3 hp. Usually the stock intake provided enough air for a stock engine. You don't really start getting any gains on the intake side until you improve exhaust flow enough that the factory intake becomes a restriction.
Usually a sportier design like the G35 has a better stock design. Case in point. Same K&N intake on an Acura TL with a 3.2 V6 shows a 7 hp advertised gain while it's 10 hp (again, advertised claims) on the Accord V6 with the 3.0. Why? The TL, like the G35 has a better intake design that takes cooler air from the front of the car and gives it a more direct path to the throttle body than the Accord.
If your car is otherwise stock, just not much to be gained (there is a huge intake dyno thread here: http://panoramic-3d.ru/forums/intake-...s-results.html
) other than some sound. Best combo for stock would be a Z-tube and an upgraded (cleanable) panel filter for the stock intake.